Two problems in my 1996 SE!! - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 12-13-2005, 10:47 PM   #1
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Default Two problems in my 1996 SE!!

Ok, so here'* the deal.

I don't know if I've been being more paranoid lately or what, but I've been noticing some changes in the way my car drives. I'd like to know a few things:

1. When I accelerate my car seems to knock. It'* a very suddel knock. If i'm coasting around 45 and I accelerate up to 50 for example, the car slightly jolts and you can hear a faint knocking sound... I'm hoping this isn't a bad sign of things to come?

2. There sounds like there is a whistling noise coming from the BACK of the car when I accelerate... I just noticed it today when I was driving and had my radio off... so maybe it'* just a normal noise and I'm being a hypchondriac. But it literally sounds like someone is quietly whistling in my trunk... and I did check for stashed bodies... no go.

Mainly I'm most concerned with problem 1 - could this be a transmission problem or something related to a spark plug or serpentine belt? I do know I should be replacing my serp belt soon and I haven't checked my spark plugs since I got the car. Anyway, I'd appreciate some help here.

Thanks alot,

Chris
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Old 12-13-2005, 10:59 PM   #2
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Default Re: Two problems in my 1996 SE!!

Quote:
Originally Posted by LBGChris
Ok, so here'* the deal.


1. When I accelerate my car seems to knock. It'* a very suddel knock. If i'm coasting around 45 and I accelerate up to 50 for example, the car slightly jolts and you can hear a faint knocking sound... I'm hoping this isn't a bad sign of things to come?
Trying to think of something cheap to explain your subtle knock, but rod bearing keeps coming to mind. How many miles are on the engine? Do you know the oil change history?

OK, here is a cheap possibility - loose flex plate bolts. Not likely, but cheap.
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Old 12-13-2005, 11:06 PM   #3
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Well this is an expensive possiblity nowadays -
But before worrying, use better gas next fill up.
That cheap stuff or any octane below 87 is just not good for a V6.

If you are already using good gas and not the crap gas, then perhaps pull one of the spark plugs and see if it is in at least OK shape.

That was problem #1, Now for #2 -

Is the seal for the trunk or back doors torn or obstructed (like maybe a wire or something unsealing the trunk)? That can cause some whisling, especially when it is really cold (think of freezing nights when the wind howls)
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Old 12-13-2005, 11:17 PM   #4
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that "knock" could be small misfires. how new are your plugs/wires?

I had small missfires out the a$$ long before it was strong enough to throw a CEL.
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Old 12-14-2005, 01:50 AM   #5
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My engine is a relatively new FRAM engine (3/4 years old). The engine itself probably has around 30,000 miles, the car has about 120,000 miles. I had to replace the engine when I got the car because the lower motor was damaged.

I'm not sure how old my plugs and wires are, would they have been replaced with the new engine as well?

I always use 87 unleaded in the 3800.

As far as the seals go, I'm not really sure -- that'* probably what'* happening. I guess I'll have to go take a look at the seals tomorrow morning.

Thanks.

Oh... and the oil change history is great -- however there was a problem with the oil sending unit and apparently the car was running without any oil in it for a small amount of time. When I took it in to the mechanic they repaired the engine but said there may be some long term damage within three months of the repair. Could this be the problem? I 'm hoping I squeaked by without any major damage.
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Old 12-14-2005, 09:54 AM   #6
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Quote:
Originally Posted by LBGChris
however there was a problem with the oil sending unit and apparently the car was running without any oil in it for a small amount of time. When I took it in to the mechanic they repaired the engine but said there may be some long term damage within three months of the repair. Could this be the problem? I 'm hoping I squeaked by without any major damage.
Not encouraging. How long did it run without oil? This could easily explain a rod knock in a relatively new engine.

It can be difficult to diagnose engine noises when you are standing beside the engine, over the internet is a little more difficult.

The noise you typically hear from detonation (too low octane) is kind of a light clattering sound high in the engine that occurs when the engine is under load.

A rod knock is a little lower in frequency and originates lower in the engine.

"Rod knocks are loudest at higher speeds (over 2500 RPM) Feathering the gas pedal may result in a distinctive back rattle between 2500 and 3500 RPMs." (Dave from remanufactured.com)

Generally speaking, the lighter and higher the sound, the less $ and trouble.

I remember a sign in a motorcycle shop:

tick..tick..tick......$50
tap..tap...tap......$150
clack...clack.......$300
knock...knock....$500
thock..thock......$1000
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Old 12-14-2005, 10:15 AM   #7
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I'm with Bill on this one.

At first I thought...that slight knock could be trans mount if you are accelerating and it'* not constant. Once I saw the no oil issue, how is the oil pressure when the car is warmed up.
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Old 12-14-2005, 11:16 AM   #8
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Some more information to help you diagnose the noise.

Spark knock (detonation) will appear as a random clattering sound with no particular rhythmic pattern. It will be loudest under load.

Rod knock, and other worn bearing noise like cam bearings, balance shaft bearing, and main bearings, will all appear as a rhythmic sound that increases and decreases with engine speed.
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Old 12-19-2005, 05:09 PM   #9
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I think I forgot to mention that I can also feel the knocking through the floorboard. The car shifts slightly...

I've also noticed that my steering wheel shakes a little more than it used to when I'm driving, is this normal in older cars?

The "knock" occurs at any time when I am cruising or accelerating -- however I usually notice it most when I am accelerating. The knock isn't constant, but happens several times during a drive. The knock usually seems to happen at speeds above 35/40MPH?

The oil pressure is fine after I got the oil sending unit repaired, according to the needle.

I'm not quite sure how long the car was without oil. My check engine light was on because of the broken oil sending unit.

I don't think the problem has anything to do with fuel.


Hope this is more clear. Thanks again.
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Old 12-19-2005, 11:41 PM   #10
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Quote:
Originally Posted by LBGChris
I think I forgot to mention that I can also feel the knocking through the floorboard. The car shifts slightly...

I've also noticed that my steering wheel shakes a little more than it used to when I'm driving, is this normal in older cars?

The "knock" occurs at any time when I am cruising or accelerating -- however I usually notice it most when I am accelerating. The knock isn't constant, but happens several times during a drive. The knock usually seems to happen at speeds above 35/40MPH?

The oil pressure is fine after I got the oil sending unit repaired, according to the needle.

I'm not quite sure how long the car was without oil. My check engine light was on because of the broken oil sending unit.

I don't think the problem has anything to do with fuel.


Hope this is more clear. Thanks again.
If the sound of the knock is evenly spaced and increases in speed with the engine, that would be characteristic of excessive clearance on a bearing, probably a rod bearing. That you can feel it through the floor indicates low frequency, like a main bearing. If you have a bearing that has spun or is going south on you, you may see little sparkly stuff in the pan when you change the oil. This would be bits of bearing material.

If the knocking sound is more of a random clattering or a random knock, that could be cheaper things, like detonation or a cylinder misfiring.

Don't mean to upset you, but you have so far described symptoms consistent with bearing failure. Although, with bearing failure, you would also usually see some reduction in oil pressure too.
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